This invention relates to cathode ray tubes employed as light sources.
Heretofore, various light source lamps, small monochromatic cathode ray tubes or the like have been employed as light source tubes for display illumination. The luminance of the light source lamps is insufficient, and the service lives thereof are relatively short. Thus, the maintenance of the light source lamps is rather troublesome. In small monochromatic cathode ray tubes, an electron beam emitted from a sealed electron gun is deflected to cause the flourescent screen to emit light. Therefore, the small monochromatic cathode ray tubes are disadvantageous in that the provision of an electron beam deflecting circuit is required. This makes the drive circuit intricate, and it is considerably difficult to simultaneously drive a plurality of small monochromatic cathode ray tubes.
FIG. 1 is a schematic sectional view of a conventional three-electrode type electron gun showing the positional relationship between the electron gun and the fluorescent screen 5 of the cathode ray tube which contains the electron gun. The electron gun includes a cathode 4, a first grid 1, a second grid 2 and a third grid 3.
An electron beam 6 emitted from the cathode 4 having an electron emitting material is controlled by a voltage E.sub.C1 applied to the first grid 1. The electron beam 6 thus controlled is accelerated by a voltage E.sub.C2 applied to the second grid 2 and is further accelerated by a voltage applied to the third grid 3, so that the beam strikes a fluorescent screen 5 which consequently emits light. The fluorescent screen 5 is so connected (not illustrated) that the potential of the screen 5 is equal to the potential E.sub.C3 of the third grid 3. A hole 0.5 to 1 mm in diameter is cut in the portion of the first grid 1, which confronts the cathode 4. Similarly, a hole 0.5 to 1 mm in diameter is cut in the portion of the second grid 2 which confronts the hole of the first grid 1.
The confronting openings of the second and third grids 2 and 3 constitute cylindrical electrodes which form an electron lens. With this arrangement, the current I.sub.K of the electron beam 6 will vary as the voltage E.sub.C1 of the first grid 1 is varied, and the diversion of the electron beam 6 is suppressed by the cylindrical electron lens formed by the second and third grids 2 and 3 so that the electron beam 6 advances to the fluorescent screen as shown, as a result of which a circular optical spot appears on the fluorescent screen 5. The diameter of optical spot is represented by D in FIG. 1.
FIG. 2 is a graphical representation indicating the relationships between the currents I.sub.K of electron beams emitted from the electron gun shown in FIG. 1 and the diameters D of optical spots on the fluorescent screens of the cathode ray tube.
In a device as shown in FIG. 1, the optical spot diameter D will change with the distance between the fluorescent screen 5 and the second grid 2. Therefore, the distance therebetween is fixed. In addition, the voltage E.sub.C2 is also set to a certain value. Under this condition, let us consider the optical spot diameter D in the case where the electron beam current I.sub.K is I.sub.KO .sup.(I.sub.K =I.sub.KO). When the fluorescent screen voltage E.sub.C3 is E.sub.a, D=D.sub.a ; when E.sub.C3 =E.sub.b, D=D.sub.b ; and when E.sub.C3 =E.sub.c, D=D.sub.c, where E.sub.c &lt;E.sub.b &lt;E.sub.a. In other words, as the voltage of the fluorescent screen 5 is decreased, the optical spot diameter D is increased; and as the fluorescent screen voltage E.sub.C3 is increased the diameter D is decreased.
The luminance of the optical spot may be increased by increasing the fluorescent screen voltage E.sub.C3, but in such a case the optical spot diameter D is decreased. Further, if the current I.sub.K is small (for instance 0 to 50 .mu.A), it may be impossible to obtain a sufficiently large optical spot diameter D even if the fluorescent screen voltage is decreased. The ratio (D/I.sub.K) of an optical spot diameter D to an electron beam current I.sub.K is generally determined by the coating material forming the fluorescent screen and the fluorescent screen voltage, and the cathode ray tube should be used in such a manner that the density of the electron beam current is smaller than the maximum permissible current density for the fluorescent screen.
As is apparent from the above description and from FIG. 2, if the fluorescent screen voltage is decreased excessively, while decreasing the fluorescent screen voltage to obtain a required optical spot diameter D, then the luminance of the optical spot is decreased to the extent that the optical spot is no longer visible. The cathode ray tube is then useless as the light source. On the other hand, if the fluorescent screen voltage is maintained high, the optical spot diameter D may be set to a required value by increasing the distance between the fluorescent screen and the electron gun, but this method is not practical because it is necessary to excessively increase the length of the light source cathode ray tube.